Process for heat-treating metals in a space containing a non-oxidizing protective gas atmosphere



06t- 22, 19 7 H. HOHN -ET AL 2,810,667

PROCESS FOR HEAT-TREATING METALS IN A SPACE CONTAINING A NON-OXIDIZING PROTECTIVE GAS ATMOSPHERE Filed, Oct. 12 1955 2 Shets-Sheet l Jig. 2

Oct. 22, 1957 H. HOHN EI'AL 2,810,667

PROCESS FOR HEAT-TREATING METALS IN A SPACE CONTAINING A NON-OXIDIZING PROTECTIVE GAS ATMOSPHERE Filed Oct. 12, 1955 2 Sheets-Sheet 2 United States Patent @fitice Patented Oct. 22, 195 7 PROCESS" FOR HEAT-TREATING METALS IN A SPACE CONTAINING A NON-OXIDIZING PRO- TECTIVE GAS ATMOSPHERE Hans Hohn, Erich Fitzer, andHeinricli Miiller, Vienna, Austria, assignors to Siemens 8;" Halske Gesellschaft m. b. H'., Vienna; Austria, :1 firm Application October 12, 1953, Serial No. 385,594

Claims priority, applicationAustria'October 14, 1952 1 Claim. (Cl; 14$13.1)

This invention relates toa process for heat-treating, more particularly annealing, sinterin gor melting metals in a space containinga non-oxidizing protective atmosphere; In this sp'ecificationand the appended claim the term heat treatment is-understood in the broadest sense.

As the development'of' metallurgy and the production of new service metals and alloys proceeds, commercial metallurgy-is increasingly required toperform thevarious heat treatments of metals; such as annealing, sintering, melting, etcain protective gas atmospheres, to prevent chemical reactions such as oxidation, nitriting, etc., and adsorption of gas Protective gases-used include, e. g., hydrogen, nitrogen, carbon monoxide, carbon dioxide, inert gases, crackedammonia, etc. Vacuum'itself is employed only inrar'e'cases, primarily only whenit is desired to degasify the metals, particularly the metal melts. To avoid theaforementioned chemical reactions it is necessary in most cases that the protective gases be subjected to prior cleaning because they always contain residual gases such as oxygen. Nevertheless that'gascleaning often is not sufficient for particularly susceptible metals.

For this reason, e. g., the sinteringof aluminum or silicon powder has been very difiicult so far, and almost impossible without additional measures" such as the'use of getters and the like. The technical vacuum'too contains an excessive amount of residual gases preventing the sintering of the metals mentioned for illustration.

According to the invention an excellent protective gas atmosphere is obtained by the use of mercury vapor'as a. protective atmosphere for the heat'treatment of metals or alloys. That measure does not only avoid the aforesaid disadvantages of the protective gases conve'ntionalso far, but achieves a number of decisive advantages.

In the first place mercury vapor does not require a gas cleaning step. Relatively small amounts of mercury are sufficient to operate even extremely large furnace units because the mercury can be recovered continuously by condensation. The useof mercury vapor protective atmospheres renders the gas supply independent of storage tanks or complicated protective gas generators, such as electrolyzers. This makes the mercury vapor protective atmosphere not only reliable in operation but inexpensive too.

The protective gas atmosphere may be produced by evaporation in the furnace itself, or in a separate evaporator. When the mercury vapor, of higher specific gravity, flows out through constrictions at the apex of the furnace space it urges all gas residues out of the furnace; they may be removed by suction after the mercury vapor has been condensed. After being cooled in a condenser the mercury is recycled to the evaporator.

Furnaces may be operated with mercury vapor protective gas even without prior evacuation or flushing with other protective gases. Whereas mercury is oxidized by oxygen at rising temperatures, mercuric oxide is decomposed quantitatively to form mercury vapor and oxygen at temperatures above 400 deg. C. In that case mercuric oxide will deposit only in-front of' the condenser in the temperature zone below 400 deg. C. Such mercuric oxide, howeven can' be reducedeasily. in a-secondary cycle with carbonmonoxide, hydrogen, etc. Any water of condensation which may be formed by the reduction is no disturbance because it stratifies in the mercury sump above themercury. In that case the metal to be heat-treated must'be stable against chemical reaction up to the temperature'at which the oxide is decomposed (40 0 deg. C.)

On principle it is -not'essential how the furnaces operated'with mercury vapor as a protective atmosphere are heated. This can be eff'ected'in a manner known per se, e. g., by a directheating of-thematerial to be annealed, e. g., resistance heating, induction heating, or by indirect heating, such as by heat radiation, heat conduction, etc. Now a most advantageous and efficient method has been found for indirectly heating the materialto be annealed. To this end the mercury vapor is superheated and in that condition is introducedinto the annealing space. Superheating the mercury vapor may be effected in various ways. E. g., the mercury vapor may be conducted through heatedmetal tubes (is. g. scale-resistant steel tubes) or allowed to pass over directly heated incandescent bodies, such as tungsten or molybdenumheating conductors, or passed through an electric are or another electric discharge gap.

It has been found that'this type of'heating with superheated mercury vapor gives most' excellent results in the removal of mercury from mercury-bearing alloys, e. g., in the evaporation (distilling) of mercury out of amalgams., In this case the metal to be heat-treated may be introduced in the form of an amalgam. In this special application the mercury to be expelled may be used both as a'protective gas and as a heat transfer medium.

Advantageously a rough vacuum is applied at the beginning of the furnace operation with mercury vapor protective gas, in order to remove quickly the major part of the atmosphere. The evaporation of mercury is known to commence in a rough vacuum at much lower temperatures (30 mm. Hg=218 deg. C.) so that a chemical reaction of the metal with the atmosphere is safely avoided. During operation, evacuation of the mercury condenser from time to time is recommended too, particularly in the case of gas-delivering metal melts. This enables in addition to the formation of a protective gas a degasification to an extent not achievable in the known vacuum furnaces. Moreover, a flushing step with known protective gases, e. g. hydrogen, may be employed at the beginning of theoperation with protective gas. This enables a removal, e. g., of residual oxygen in the quickest way on the principle described.

The invention will be explained more fully hereinafter with reference to the accompanying drawings, in which examples of the application of the process according to the invention, and of apparatus for carrying it out, are shown,

Fig. 1 representing a'hocdtypeannealing furnace,

Fig. 2 a tilting melting furnace with graphite, electric are or induction heating,

Fig. 3 an annealing furnace for piece material, which is to be fed and discharged through a lock,

Fig. 4 a continuous furnace, and

Fig. 5 a muille furnace with a heating coil, e. g. of tungsten or molybdenum, which is susceptible to oxidation.

In Fig. 1, 1 is the furnace hood, 2 the furnace heater, 3 the gas-sealing furnace bottom, 4 designates the material to be annealed, 5 is the mercury for producing the protective atmosphere. The mercury is connected through a tube 6 with the condenser 7, the other end of which communicates through tube 8 with the furnace atmosphere.

3 Thus the mercury cycle is closed between the furnace and the condenser. Through pipe 8 the interior of the furnace communicates through valve 9 with a vacuum pump and through valve 10 with the outside air. 11 is a heater which may be provided additionally for the mercury sump in the furnace.

When the furnace has been filled with material to be annealed and closed tightly, the mercury heater is operated while a rough vacuum is applied at the same time. As soon as the mercury passes through the condenser, the furnace heater is turned on; then the rough vacuum may be disconnected and the annealing operation may be performed. After the annealing treatment has been terminated, it is recommendable to turn off the mercury heater 11 before turning off the furnace heater 2. Thereby a removal of mercury from the material annealed is effected. After the furnace has cooled down and has been aerated through the cock 10 the furnace may be opened. In many cases a separate, additional mercury heater may be eliminated, the evaporation of the mercury being effected by the furnace heater itself.

In Fig. 2, 21 designates the furnace shell connected to a heatable chill mold 22, whose heater is indicated at 23. The furnace and chill mold are flushed continuously with mercury vapor from a mercury evaporator 24. 25 is the condenser for condensing the mercury vapor. Through valve 26 a constantly running vacuum pump is connected, for complete degasification of the metal melt. Pouring is effected by tilting the furnace into the chill mold also filled with mercury vapor.

In Fig. 3 the interior 31 of the furnace, with the heaters 32, is connected in the mercury cycle, the mercury to be evaporated being supplied to the interior of the furnace through a tube 33 and in riser tubes 34. The lock of the furnace is designated with 35, the condenser, which is arranged outside the furnace, with 36. In the riser tubes 34 the mercury may be conveyed, e. g., to advantage on the electrodynamic principle, the current flow through the mercury supplying at least part of the heat required for evaporation. To advantage the heating space of the furnace and the lock may be evacuated before they are flushed with mercury vapor.

Furnaces operated with mercury vapor protective gas are most suitable particularly for continuous operation because the condensed liquid mercury provides an excellent gas seal for the material feed and discharge points. Whereas the material may be fed through the mercury in any case, the discharge of the material through the mercury is restricted to practically non-amalgamable metals such as iron, nickel, chromium, tungsten, molybdenum, and their alloys. It has been found, however, that with the other metals, such as aluminum, copper, and others, the amalgamation occurring at the discharge point is even desirable in certain cases, e. g. for improved shaping and subsequent processing, in single cases even for protection against corrosion.

The application of such mercury barrier is shown in Fig. 4 in a continuous furnace, which may be used to advantage, e. g. for annealing rolled stock such as strip, sections, etc., or for hot-working wires or strip, or especially for the sintering treatment of compressed metal powder. 41 is the heating section proper of the furnace with the heating coils 42. For carrying the material to be treated in the furnace an endless belt 43 is provided with annealing boxes 44, which are filled at 45 with material to be sintered and emptied automatically at 46.

The evaporation of mercury to form the protective gas is effected in this case from the bottom of the furnace by the furnace heat itself. The cooled parts 47 of the furnace serve as mercury condensers supplying the underlying furnace bottom with liquid mercury. The feeding and discharge of the material is effected through mercury barriers at the feed and discharge openings 48 and 49, respectively. These barriers at the same time provide the gas seal of the interior of the furnace against the outside atmosphere. At the discharge point a layer of water, glycerine or similar liquids, which may be slightly acidulated, may be placed above the mercury to remove more effectively from the material treated in the furnace any adhering mercury particles. If required a vacuum may be connected additionally at 50.

Referring to Fig. 5, the muffle 52, heated from the outside, is accommodated in the gas-sealed furnace space 51 having a highly refractive lining. 53 designates the closure of the mufile towards the outside. Thus the muffle 52 defines in the furnace space 51 a heater compartment outside the mutfie and a compartment for the metal to be treated inside the muffle. Consisting, e. g. of tungsten or molybdenum the heating coil 54 arranged in the heater compartment is susceptible to oxidation.

The heating coil 54 is swept by a stream of mercury vapor, which is conducted through the tube 55 in a cycle in which is connected the condenser 56 arranged outside the furnace. The evaporation of the mercury is effected at the furnace bottom by the heating coil itself. Suitably an additional mercury evaporator may be provided to prevent even the initial oxidation of the heating coil susceptible to oxidation.

We claim:

A process for heat-treating metals in a furnace space, which comprises heating in said space an amalgam consisting of mercury and a metal to be heat-treated to distill the mercury from said amalgam and thus to maintain References Cited in the file of this patent UNITED STATES PATENTS 279,840 .Stetefeldt June 19, 1883 843,563 Frith Feb. 5, 1907 1,688,481 Austin et al. Oct. 23, 1928 2,418,088 Nachtman Mar. 25, 1947 2,618,550 Hampel et al Nov. 18, 1952 

